Source : Webster's Revised Unabridged Dictionary (1913)

Castrato \Cas*tra"to\, n. [L., properly p. p. of castrare. See
   {Castrate}.]
   A male person castrated for the purpose of improving his
   voice for singing; an artificial, or male, soprano. --Swift.

Source : WordNet®

castrato
     n : a male singer who was castrated before puberty and retains a
         soprano or alto voice
     [also: {castrati} (pl)]